Fabric-creping has been employed in connection with papermaking processes which include mechanical or compactive dewatering of the paper web as a means to influence product properties. See U.S. Pat. Nos. 4,689,119 and 4,551,199 of Weldon; 4,849,054 and 4,834,838 of Klowak; and 6,287,426 of Edwards et al. While in many respects, these processes have more potential than conventional papermaking processes in terms of energy consumption and the ability to use recycle fiber, operation of fabric-creping processes has been has hampered by the difficulty of effectively transferring a web of high or intermediate consistency to a dryer. Note also U.S. Pat. No. 6,350,349 to Hermans et al. which discloses wet transfer of a web from a rotating transfer surface to a fabric. Further United States Patents relating to fabric-creping more generally include the following: U.S. Pat. Nos. 4,834,838; 4,482,429; 4,448,638 as well as 4,440,597 to Wells et al.
More recently, high-speed fabric-crepe processes have been developed as is seen in U.S. application Ser. No. 10/679,862, filed Oct. 6, 2003, entitled “Fabric-crepe Process for Making Absorbent Sheet”; GP-02-12). The level of adhesion of the papermaking web to a Yankee dryer cylinder is of importance as it relates to transfer of the web from a creping fabric to the drying cylinder, as well as control of the web in-between the dryer and the reel upon which a roll of the paper is being wound. Webs which are insufficiently adhered may blister or, even worse, become disengaged from a drying cylinder and cause a hood fire.
Moreover, wet-tack is critical in fabric-crepe processes where insufficient wet-tack may lead to a transfer failure wherein the web fails to transfer from a creping fabric to a drying cylinder and remains imbedded in a fabric causing shutdowns and waste of material and energy.
Further, the level of adhesion of the papermaking web to the dryer is of importance as it relates to the drying of the web. Higher levels of adhesion reduce the impedance to heat transfer and cause the web to dry faster, enabling more energy efficient, higher speed operation; provided excessive build-up of adhesive is avoided. Note, however, that some build-up is desirable inasmuch as adhesion of the sheet to the dryer occurs largely by means of creping adhesive deposited in previous passes. Thickness of a coating layer on a Yankee drying cylinder tends to increase with time, insulating a wet web from the Yankee surface to the web. In other words, the adhesive coating build-up on the Yankee reduces heat transfer from the Yankee surface. To maintain the same moisture level in the finished product, the Yankee hood temperature (and energy input to the web) is increased accordingly. After a production interval of two hours or so, the hood temperature reaches its upper ceiling and the coating layer needs to be stripped off to reduce the hood temperature to a normal operating window. A new cleaning doctor is typically used to strip off the old coating build-up.
Stripping of the coating, however, results in sheet transfer problems at the pressure roll due to blistering and edge floating. Further details are seen in copending U.S. Provisional Patent Application Ser. No. 60/779,614, entitled “Method of Controlling Adhesive Build-Up on a Yankee Dryer”, filed Mar. 6, 2006; GP-06-1), the disclosure of which is incorporated herein by reference.
Even if the stripping operation is accomplished efficiently, downtime reduces production significantly.
Initially, operation of high-speed fabric-crepe processes was based, in part, on the belief that the wet-tack required for effective transfer from a creping fabric to a Yankee drying cylinder was best achieved with relatively wet sheet and relatively high levels of creping adhesive, especially a hygroscopic re-wettable adhesive such as polyvinyl alcohol resin.
It has been unexpectedly found in accordance with the present invention that low levels of creping adhesive on a Yankee drying cylinder are advantageously employed in a production process with a heated cylinder upstream of the Yankee.